In a digital light processing (DLP) device that uses a micro-mirror array to cast an image onto a screen, an arrangement of high-power light-emitting diodes (LEDs) can be used as a light source. Apart from their favorably long life-time, an advantage of using LEDs is that they can be switched on and off at very high switching frequencies. Furthermore, the light output essentially follows the current though the LEDs, so that the light output is also characterized by favorably fast rise and fall times. To operate a DLP device, control signals are issued to the micro-mirror array to tilt each micro-mirror according to its contribution in the image being generated. Essentially simultaneously, control signals are issued to the LEDs to regulate the light output according to the image being generated. For example, control signals are issued according to the amount of red, green and blue light required for each pixel of the image, and according to the light intensity of each pixel. A requirement of a power converter supplying current and voltage to the LED load is that the current should be characterized by low overshoot or undershoot and highly accurate on/off timing.
In most such applications, a switched-mode power supply (SMPS) is used to drive the LEDs, which are connected as a load across an output filter of the SMPS. The output filter is generally realized using a capacitor or a pi-filter. The SMPS can provide an output power that is switched rapidly between a high level (“on”) and a low level (“off”) in a so-called hysteretic mode of operation. Because of the desired high switching frequency, a field-effect transistor (FET) is a suitable choice of switch for this purpose. In this “active” mode, an essentially constant output voltage is provided at the output terminals of the converter, with a certain amount of ripple. A control processor of the micro-mirror array can issue appropriate control signals to the converter that are synchronized with control signals of the micro-mirror array. By generating an appropriate gate signal for the converter FET switch, the converter output can alternate between “active” and “inactive” phases. In this way, a desired level of dimming can be achieved for each color of the LED arrangement according to each pixel of the image array. In some known applications, the lighting load itself can be switched independently of the SMPS. To this end, a switch usually a semiconductor switch can be added in series with the lighting load to turn this on and off as required. Such a control signal is generally referred to as a strobe signal. Here also, a field-effect transistor (FET) is a suitable choice of switch for turning the LED load on and off. In a conventional arrangement, gate control signals for the converter switch and load switch are generally issued simultaneously. However, this can result in a noticeable overshoot (or undershoot) on the LED current, which in turn leads to an overshoot (or undershoot) on the light output. Excess light as a result of an overshoot can be perceived by a viewer, thus detracting from the quality of the viewing experience. In a projection arrangement using such a driver, the micro-mirror timing may have to compensate for an overshoot/undershoot. However, some light is effectively wasted, leading to a reduction in the overall efficiency and brightness.
Therefore, it is an object of the invention to provide an improved way of controlling LED light sources, particularly in a DLP projection arrangement.